1. Field of the Invention
The invention relates generally to magnetic data storage drives, and more specifically to a method and an apparatus for minimizing the effects of noise sources through the utilization of oversampling techniques.
2. Description of the Prior Art
State of the art data storage drives often utilize magnetic data storage media such as tapes and/or disks. These drives employ magnetic read/write heads to perform data access operations on the data storage medium. For example, in the context of disk media, magnetoresistive (MR), thin film, or ferrite (MIG) heads are often employed. Although these read/write heads have provided generally satisfactory results for some system applications, the use of these heads may present significant drawbacks. Read/write heads, especially the magnetoresistive (MR) type, are very susceptible to stray electromagnetic fields emanating from sources other than the disk media. Stray electromagnetic field pickup is undesirable because it adds noise to the signal read by the MR head. If the stray fields cause a significant degradation in signal to noise ratio, data may be lost and/or altered.
One approach for reducing the noise pickup of the read/write head, applicable in the context of MR heads, is to ensure that the shields on either side of the MR head stripe are isolated from the slider body. The connection of one of these shields to the slider body destroys the noise immunity afforded by common mode rejection, thereby rendering the head susceptible to a multitude of noise sources from within the disk drive. The head will also be vulnerable to fields originating outside of the disk drive.
Two major factors cause the shield to short out to the slider body. First, the short may occur during the manufacturing process. Second, even if the head leaves the factory with complete shield isolation, head-to-disk contact may cause a short to occur. These factors are often beyond the control of the head manufacturer and the end user. As a practical matter, it is difficult or impossible to ensure that the MR head shields are completely isolated. Therefore, efforts to improve the shielding of the head may prove futile. Efforts should instead be directed to improved signal processing methods which have the potential for substantially reducing readback signal noise.
One prior art technique for reducing noise in the readback signal is the use of a sampling process in conjunction with a digital filter. A block diagram of such a system is shown in FIG. 1. The analog readback signal received by transducer 39 is amplified by amplifier 101 and fed to the input of a high bandwidth analog lowpass filter 102 to remove high frequency noise from the readback signal. The lowpass-filtered signal is then passed on to an automatic gain control (AGC) 103 circuit which output is connected to an analog-to-digital converter (ADC) 104. The ADC 104 samples the incoming signal at periodic intervals spaced apart in time by To. The digital signal samples are fed to a partial response digital filter (PRDF) 106. The PRDF 106 serves as an equalizer and has a frequency response providing a relatively high gain over a range of mid-band frequencies, with tapered lower gain at both frequency extremes. The PRDF 106 may be, for example, a response class IV finite impulse response (FIR) filter. The digitally equalized filter output 119 is then ready to be decoded by decoder 108. Decoder 108 could be, for example, a Viterbi decoder cascaded with a run length limited (RLL) decoder. Decoder 108 also provides digital output synchronization lines 141 and 142 that are digital to analog converted by DACs 109 and 110, respectively. The decoder 108 recovers the data from the readback signal and sends it to the host computer on line 140.
The synchronization of AGC 103, ADC 104, PRDF 106, and digital decoder 108 is performed by a voltage-controlled oscillator (VCO) 113. The VCO 113 operates under feedback control from the analog output line 143 of DAC 109. The DAC 109 derives the feedback signal from the digital decoder 108 over the digital output synchronization line 141. The VCO output 120 provides a timing signal that is synchronized and locked to the readback signal. The VCO 113 uses the disk drive system crystal reference oscillator (OSC) 112 for bootstrapping and frequency reference purposes. The AGC 103 is controlled by a feedback signal from the digital decoder 108 over analog line 144.
The frequency of the VCO 113 determines the interval at which samples of the readback signal are to be taken. The VCO 113 provides the analog-to-digital converter 104 with a periodic waveform which controls the opening and closing of an electronic switch 117 within the ADC 104. The ADC 104 samples the signal passing through the electronic switch 117 and arriving at the input port 115 of the PRDF 106. Accordingly, the VCO 113 will adjust its frequency until signals at the desired sampling rate appear at the input port 115 of the PRDF 106. The output of the VCO 113 is also used to provide a synchronization signal for the digital decoder 108 at the decoder sync 121 input.
The system of FIG. 1 is subject to various design constraints. The theoretical minimum sampling rate for To is the bit rate or the Nyquist rate of the readback signal. However, when sampling the readback signal from the recording head 39, the sampling rate should be at least twice the highest expected data rate to avoid noise aliasing into the bandwidth of the recording channel. For example, if the highest data frequency is 10 MHz, and the sampling rate is 30 MHz, noise in the readback signal in the range of 18 to 22 MHz would alias down into the baseband from 8 to 12 MHz.
Noise aliasing significantly limits the performance of prior art disk drive systems employing a sampling process, such as the system shown in FIG. 1. Noise aliasing is of particular concern in state of the art disk drive systems, because a major design objective of these systems is to provide a data rate which is as high as possible. The wide data bandwidth of such disk drive systems increases the potential susceptibility of these systems to noise. Noise aliasing results in the transformation of high frequency noise down into the bandwidth of the data signal. This in-band noise is indistinguishable from the data readback signal, and is therefore impossible to remove.
The system configuration of FIG. 1 places the analog lowpass filter 102 and AGC 103 before the ADC 104 in the signal chain so as to reduce the noise aliasing problem. However, due to the proximity of the data frequencies to the noise frequencies, the lowpass filter 102 must have a relatively steep cutoff. Any phase nonlinearities in the filter will corrupt the readback signal.
Using state of the art analog filter design techniques, it is very difficult to design a practical, economically feasible lowpass filter 102 which has a frequency response providing a steep cutoff, while at the same time providing linear phase characteristics and reasonably low insertion loss. Bessel filter designs provide high insertion loss, but offer the advantages of comparatively sharp frequency cutoff characteristics and good linear phase response relative to other types of filter designs, such as the Chebychev, Butterworth, and Elliptic classes of filters. However, Bessel filters are very expensive to design and fabricate. A Bessel filter constructed to meet the requirements of the circuit shown in FIG. 1 would necessitate the use of many cascaded filter stages. A further disadvantage of Bessel filters is that they are overly sensitive to parameter variations at the input and output ports. The Bessel filter parameters will exhibit unacceptable variations from disk drive to disk drive. Even if the filters are optimized for use with a specific disk drive, the parameters also exhibit unacceptable variations across temperature. Accordingly, Bessel filters are not well suited for application as anti-aliasing lowpass filters in disk drive systems using read/write heads.
At present, there are no suitable alternative filter designs which would overcome the shortcomings of the Bessel filter configuration. The major disadvantage of the remaining filter design configurations relates to nonlinear phase response versus frequency. The nonlinear phase characteristics of alternative steep-cutoff lowpass filter designs cause phase distortions, such as zero-crossing distortion, in the higher frequencies of the readback signal. This type of distortion is often referred to as timing or crossover distortion, and it causes data errors in the detection process.
Design tradeoffs are inherent in Elliptic filter (sharp frequency cutoff) and Bessel filter (linear phase response) designs. Sharp frequency cutoff and linear phase response are mutually exclusive in simple analog filters. This design tradeoff can only be overcome through the utilization of relatively complex cascaded analog filters which are not cost-effective.
As with the lowpass filter 102, the PRDF 106 should also be designed to provide a minimum of phase distortion. However, unlike the situation with analog filters, it is a relatively straightforward matter to design a suitable PRDF 106 having linear phase response. The design of a suitable PRDF 106 for use in the configuration of FIG. 1 is well known to those skilled in the art.
The conventional sampling apparatus of FIG. 1 presents an additional shortcoming. The current trend is towards high bandwidth data channels, which increases the sampling rate required to reduce noise. However, under current state of the art technology, the maximum sampling rate is limited by the cutoff frequencies of existing solid-state devices. For high bandwidth channels, it would not be cost-effective to provide the high-frequency circuitry necessary to implement relatively high sampling rates.
FIG. 2 is a waveform showing the readback signal from an MR head using existing state of the art technology as illustrated in FIG. 1. The signal represents a CORSAIR file and has a baseband frequency of 2 MHz. The sampling rate is 20 nanoseconds, and no oversampling filtering technique is used. Of particular importance is the magnitude of the high frequency noise superimposed upon the readback signal. The noise amplitude is almost equal to the signal amplitude.
The waveform of FIG. 2 provides an illustrative example of magnetic domain noise superimposed on a data signal. This noise is especially visible in the range of 1.25 to 1.5 microseconds, as a damped high-frequency oscillation riding upon the 2 MHz data signal. Magnetic domain noise is caused by random variations in the magnetic domains of the disk occuring across an area traversed by the MR head. The noise exhibits nonlinear characteristics. In order to improve overall system performance, this noise should be reduced or eliminated from the data signal.
Accordingly, there is a manifest need for an improved noise minimization technique for data storage drives. Preferably, such a technique should not require the use of relatively high sampling frequencies, which is an especially important consideration in the context of high bandwidth channels. The technique should minimize or eliminate the problem of high frequency noise aliasing. Phase-linear filters should be employed to provide an accurate rendering of the data signal.